Typically, a solid oxide fuel cell (SOFC) employs an electrolyte of ion-conductive solid oxide such as stabilized zirconia. The electrolyte is interposed between an anode and a cathode to form an electrolyte electrode assembly. The electrolyte electrode assembly is interposed between separators (bipolar plates). In use, generally, predetermined numbers of the electrolyte electrode assemblies and the separators are stacked together to form a fuel cell stack.
In the fuel cell, in order to supply a fuel gas such as a hydrogen-containing gas and an oxygen-containing gas such as the air to the anode and the cathode of the electrolyte electrode assembly, a fuel gas channel and an oxygen-containing gas channel are formed along surfaces of the separator. The fuel cell stack may adopt internal manifold structure where a fuel gas supply passage extends through the fuel cell stack in the stacking direction for distributing the fuel gas to each fuel gas channel.
For example, in a solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 2006-269409, as shown in FIG. 11, a manifold 2a extending through a fuel cell stack 1a is provided for supplying a reactant gas through the manifold 2a. The reactant gas is supplied to each of power generation cells through gas channels 4a in separators 3a connected to the manifold 2a. 
The manifold 2a and the gas channels 4a in the separators 3a are connected in the stacking direction through throttle mechanisms 5a for limiting gas flow. According to the disclosure, the amount of gas supplied to each of the power generation cells become uniform, and stability in the cell output and improvement in the output efficiency are achieved.
Further, in a flat plate type solid oxide fuel cell disclosed in Japanese Laid-Open Patent Publication No. 10-172594, unit cells (not shown) and separators 1b are provided alternately, and as shown in FIG. 12, gas supply holes 2b and gas discharge holes 3b extend through four corners of the separator 1b in the stacking direction, and a plurality of gas flow grooves 4b and ridges 5b in a plurality of rows are arranged alternately along the surface of the separator 1b. 
The gas flow grooves 4b are connected to the gas supply hole 2b and the gas discharge hole 3b through triangular recesses 6b. A throttle section 7b and blocks 8b are provided in a gas inlet of the triangular recess 6b, near the gas supply hole 2b, as means for limiting the flow rate of the gas.
The throttle section 7b and the blocks 8b function to increase the pressure loss of the gas flowing from the gas supply hole 2b to the gas inlet for uniform gas distribution. Further, at opposite ends of the gas flow grooves 4b, shallow gas flow inlets 9b are provided for causing a pressure loss in the gas flow.
In Japanese Laid-Open Patent Publication No. 2006-269409, the manifold 2a and the gas channels 4a in the separators 3a are connected through the throttle mechanisms 5a, and variation tends to occur in the precision of machining the throttle mechanisms 5a. Further, in the throttle mechanisms 5a, the pressure loss changes depending on the actual machine state (operating state) or the inspecting state (interruption state). Therefore, the pressure loss inspection cannot be performed accurately.
Further, in Japanese Laid-Open Patent Publication No. 10-172594, in order to maintain the desired pressure loss in the gas flowing from the gas supply hole 2b to the gas flow inlet 9b, high machining precision of the throttle section 7b and the blocks 8b needs to be maintained. Therefore, the cost of producing the separator 1b becomes considerably high uneconomically.